Leupold one piece base is what mine had on it. And yes on stabilizing 100gr Partitions. Mine was one of the very first 260s produced and it shot fine with 120 Ballistic tips and numerous 140s, all of which would be longer than 100gr Partitions.
Years after when I ended up with another early M7 with only 3 base holes I simply chucked it in the mill and drilled and tapped the 4th hole. Easy job that opens up a bunch of mounting options.
